New tacrine-huperzine A hybrids (huprines): highly potent tight-binding acetylcholinesterase inhibitors of interest for the treatment of Alzheimer's disease

Several new 12-amino-6,7,10,11-tetrahydro-7, 11-methanocycloocta[b]quinoline derivatives (tacrine-huperzine A hybrids, huprines) have been synthesized and tested as acetylcholinesterase (AChE) inhibitors. All of the new compounds contain either a methyl or ethyl group at position 9 and one or two (chloro, fluoro, or methyl) substituents at positions 1, 2, or 3. Among the monosubstituted derivatives, the more active are those substituted at position 3, their activity following the order 3-chloro > 3-fluoro > 3-methyl > 3-hydrogen. For the 1,3-difluoro and 1,3-dimethyl derivatives, the effect of the substituents is roughly additive. No significant differences were observed for the inhibitory activity of 9-methyl vs 9-ethyl derivatives mono- or disubstituted at positions 1 and/or 3. The levorotatory enantiomers of these hybrid compounds are much more active (eutomers) than the dextrorotatory forms (distomers) as AChE inhibitors. Compounds rac-20, (-)-20, rac-26, (-)-26, rac-30, (-)-30, and rac-31 showed human AChE inhibitory activities up to 28.5-fold higher than for the corresponding bovine enzyme. Also, rac-19, (-)-20, (-)-30, and rac-31 were very selective for human AChE vs butyrylcholinesterase (BChE), the AChE inhibitory activities being 438-871-fold higher than for BChE. Several hybrid compounds, specially (-)-20 and (-)-30, exhibited tight-binding character, showing higher activity after incubation of the enzyme with the inhibitor than without incubation, though the reversible nature of the enzyme-inhibitor interaction was demonstrated by dialysis. The results of the ex vivo experiments also supported the tight-binding character of compounds (-)-20 and (-)-30 and showed their ability to cross the blood-brain barrier. Molecular modeling simulations of the AChE-inhibitor complex provided a basis to explain the differences in inhibitory activity of these compounds.     

Development of bivalent acetylcholinesterase inhibitors as potential therapeutic drugs for Alzheimer's disease

At present the only FDA-approved therapy for Alzheimer's disease involves the administration of acetylcholinesterase inhibitors, to alleviate the cholinergic deficit associated with this disease. However, none of the approved drugs is ideal in efficacy or tolerability. One possible strategy to improve selectivity and potency is to design drugs that can simultaneously bind to the catalytic and peripheral anionic sites of AChE. In this review we will describe the development of dimeric AChE inhibitors, from the early observations of high inhibition potency by bis-quaternary inhibitors, to the structure-based design of dimers based on tacrine, huperzine A, galanthamine, and polyamines.     

Synthesis, in vitro pharmacology, and molecular modeling of very potent tacrine-huperzine A hybrids as acetylcholinesterase inhibitors of potential interest for the treatment of Alzheimer's disease.

Eleven new 12-amino-6,7,10,11-tetrahydro-7, 11-methanocycloocta[b]quinoline derivatives [tacrine (THA)-huperzine A hybrids, rac-21-31] have been synthesized as racemic mixtures and tested as acetylcholinesterase (AChE) inhibitors. For derivatives unsubstituted at the benzene ring, the highest activity was obtained for the 9-ethyl derivative rac-20, previously prepared by our group. More bulky substituents at position 9 led to less active compounds, although some of them [9-isopropyl (rac-22), 9-allyl (rac-23), and 9-phenyl (rac-26)] show activities similar to that of THA. Substitution at position 1 or 3 with methyl or fluorine atoms always led to more active compounds. Among them, the highest activity was observed for the 3-fluoro-9-methyl derivative rac-28 [about 15-fold more active than THA and about 9-fold more active than (-)-huperzine A]. The activity of some THA-huperzine A hybrids (rac-19, rac-20, rac-28, and rac-30), which were separated into their enantiomers by chiral medium-pressure liquid chromatography (chiral MPLC), using microcrystalline cellulose triacetate as the chiral stationary phase, showed the eutomer to be always the levorotatory enantiomer, their activity being roughly double that of the corresponding racemic mixture, the distomer being much less active. Also, the activity of some of these compounds inhibiting butyrylcholinesterase (BChE) was tested. Most of them [rac-27-31, (-)-28, and (-)-30], which are more active than (-)-huperzine A as AChE inhibitors, turned out to be quite selective for AChE, although not so selective as (-)-huperzine A. Most of the tested compounds 19-31 proved to be much more active than THA in reversing the neuromuscular blockade induced by d-tubocurarine. Molecular modeling of the interaction of these compounds with AChE from Torpedo californica showed them to interact as truly THA-huperzine A hybrids: the 4-aminoquinoline subunit of (-)-19 occupies the same position of the corresponding subunit in THA, while its bicyclo[3.3.1]nonadiene substructure roughly occupies the same position of the corresponding substructure in (-)-huperzine A, in agreement with the absolute configurations of (-)-19 and (-)-huperzine A.     

Novel multipotent tacrine-dihydropyridine hybrids with improved acetylcholinesterase inhibitory and neuroprotective activities as potential drugs for the treatment of Alzheimer's disease

In this work we describe the synthesis and biological evaluation of the tacrine-1,4-dihydropyridine (DHP) hybrids (3-11). These multipotent molecules are the result of the juxtaposition of an acetylcholinesterase inhibitor (AChEI) such as tacrine (1) and a 1,4-DHP such as nimodipine (2). Compounds 3-11 are very selective and potent AChEIs and show an excellent neuroprotective profile and a moderate Ca2+ channel blockade effect. Consequently, these molecules are new potential drugs for the treatment of Alzheimer's disease.     

Tacrine-huperzine A hybrids (huprines): a new class of highly potent and selective acetylcholinesterase inhibitors of interest for the treatment of Alzheimer's disease

Tacrine-huperzine A hybrids (huprines) are a new class of very potent and selective acetylcholinesterase (AChE) inhibitors. Huprines were designed from tacrine and (-)-huperzine A through a conjunctive approach. They combine the 4-aminoquinoline substructure of tacrine with the carbobicyclic substructure of (-)-huperzine A. Structural variations on several parts of a lead structure have allowed to complete a structure-activity relationship exploration of this new structural family and have led to several huprines more active than other known AChE inhibitors.     

Protease inhibitors as potential disease-modifying therapeutics for Alzheimer's disease

The current lack of an effective treatment for Alzheimer's disease (AD) has fuelled an intense search for novel therapies for this neurodegenerative condition. Aberrant production or decreased clearance of amyloid-beta peptides is widely accepted to be causative for AD. Amyloid-beta peptides are produced by sequential processing of the beta-amyloid precursor protein by the two aspartyl-type proteases beta-secretase and gamma-secretase. Because proteases are generally classified as druggable, these secretases are a centre of attraction for various drug discovery efforts. Although a large number of specific drug-like gamma-secretase inhibitors have been discovered, progress towards the clinic has been slowed by the broad substrate specificity of this unusual intramembrane-cleaving enzyme. In particular, the Notch receptor depends on gamma-secretase for its signalling function and, thus, gamma-secretase inhibition produces distinct phenotypes related to a disturbance of this pathway in preclinical animal models. The main task now is to define the therapeutic window in man between desired central efficacy and Notch-related side effects. In contrast, most studies with knockout animals have indicated that beta-secretase inhibition may have minimal adverse effects; however, the properties of the active site of this enzyme make it difficult to find small-molecule inhibitors that bind with high affinity. In most instances, inhibitors are large and peptidic in nature and, therefore, unsuitable as drug candidates. Thus, there are many issues associated with the development of protease inhibitors for AD that must be addressed before they can be used to test the 'amyloid cascade hypothesis' in the clinic. The outcomes of such trials will provide new directions to the scientific community and hopefully new treatment options for AD patients.     

Progress in acetylcholinesterase inhibitors for Alzheimer's disease: an update

Background: To date, the pharmacotherapy of Alzheimer's disease (AD) has been based on acetylcholinesterase inhibitors (AChEIs), and more recently on an N-methyl-d-aspartate receptor antagonist. By increasing acetylcholine concentration in the brain, AChEIs slow behavioral and functional impairments, improving cognitive function. Objective: The review provides an update on novel analogs of approved AChEIs, their combination with other anti-AD agents, natural AChEIs, and modern multitarget-directed ligands (MTDLs) able to hit different biological targets. Methods: We reviewed patents filed during 2005 – 2007 dealing with new AChEIs and their potential application for AD treatment. We point out new chemical structures and scaffolds for designing new AD therapeutic agents as well as new combinations or MTDLs. Results and conclusions: Compared to the limited number of novel commercially available AChEI analogs, many new natural compounds were patented for AD treatment. These might represent a starting point for the rational design of new MTDLs.     

New classes of AChE inhibitors with additional pharmacological effects of interest for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is associated to a gradual loss of attention and memory that have been associated to impairment of brain cholinergic neurotransmission, particularly a deficit of cholinergic neurons in the nucleus basalis of Meynert. Thus, it is not surprising that the first therapeutic target that has demonstrated therapeutic efficacy on cognition, behaviour and functional daily activities has been the inhibitors of acetylcholinesterase (AChE), i.e. tacrine, donepezil, rivastigmine and galanthamine. But not all inhibitors of AChE have the same potency to block the enzyme and have a different pharmacological profile. For instance, rivastigmine is a dual inhibitor of AChE and butyrylcholinesterase (BuChE), and galanthamine is a mild inhibitor of AChE and an allosteric potentiating ligand of neuronal nicotinic receptors for acetylcholine (nAChRs). In addition, we have recently found that galanthamine has neuroprotective effects by inducing calcium signals and the induction of the antiapoptotic protein Bcl-2. In this frame, we have been synthesizing new tacrine derivatives that keep their ability to inhibit AChE but that interfere with neuronal calcium overloading and prevent apoptosis. Some of these compounds exhibit neuroprotecting properties and thus, could be useful in the treatment of neurodegenerative and ischaemic brain diseases.     

Outcomes of Alzheimer's disease therapy with acetylcholinesterase inhibitors and memantine

Introduction: Alzheimer's disease (AD) is a world-wide health problem with implications for an increasing number of people and countries. Populations suffering from AD financially strain the healthcare budgets of rich and poor countries alike. Moreover, no effective treatment is available and current drugs merely slow the progression of cognitive function deterioration and overall health status toward an inevitable end point. An increasing number of novel approaches have been tested in numerous clinical trials, but none of them has proved safe and effective for treating AD.

Areas covered: This review summarizes all currently available compounds (donepezil, rivastigmine, galantamine, memantine) for the management of AD, concentrating on clinical aspects such as the mechanisms of action, pharmacokinetics, pharmacodynamics and clinical trials. This review also considers the mechanisms and side effects to provide perspective on current treatment options.

Expert opinion: Novel approaches in the treatment of AD are being intensively tested, but so far without any major success. Patients diagnosed with AD still mostly benefit from four compounds to significantly improve cognition functions and overall health and help manage other symptoms or even prolong the symptom-free period.     

Synthesis and biological evaluation of tacrine-thiadiazolidinone hybrids as dual acetylcholinesterase inhibitors

The synthesis of tacrine-thiadiazolidinone hybrids is described. These compounds are designed as dual acetylcholinesterase inhibitors binding simultaneously to the peripheral and catalytic sites of the enzyme. All tested compounds exhibit significant AChE inhibitory activity. Competition assays using propidium as reference of selective ligand for the peripheral anionic site on acetylcholinesterase indicates the influence of the designed compounds over the peripheral site. They can be considered as new leads in the optimization of Alzheimer's disease modifying agents.     

Effects of acetyl-L-carnitine in Alzheimer's disease patients unresponsive to acetylcholinesterase inhibitors

Acetyl-L-carnitine (ALC) is a compound acting as an intracellular carrier of acetyl groups across inner mitochondrial membranes. It also appears to have neuroprotective properties and it has recently been shown to reduce attention deficits in patients with Alzheimer's disease (AD) after long-term treatment. We performed an open study to evaluate the effect of ALC (2 g/day orally for 3 months) in association with donepezil or rivastigmine in 23 patients with mild AD who had not responded to treatment with acetylcholinesterase inhibitors (AChE-I). Clinical effects were evaluated by assessing cognitive functions, functional status and behavioural symptoms. The response rate, which was 38% after AChE-I treatment, increased to 50% after the addition of ALC, indicating that the combination of these two drugs may be a useful therapeutic option in AD patients. These data do not permit a conclusion as to the possible mechanism of action of the association of the two treatments.     

Acetylcholinesterase inhibitors as disease-modifying therapies for Alzheimer's disease

The therapeutic arsenal for the treatment of Alzheimer's disease (AD) remains confined to a group of four inhibitors of AChE and one NMDA receptor antagonist, which are used to provide a relief of the very late symptoms of the dementia, i.e. the cognitive and functional decline. In line with the growing body of evidence of the pivotal role of the beta-amyloid peptide (Abeta) in the pathogenesis of AD, alternative classes of drugs targeting mainly the formation or the aggregation of Abeta are actively pursued by the pharmaceutical industry, as they could positively modify the course of AD, stopping or slowing down disease progression. While the first amyloid-directed disease-modifying drugs go ahead with their clinical development and could reach the market as soon as 2009, mounting preclinical and clinical evidences is pointing towards a disease-modifying role also for currently marketed anti-Alzheimer AChE inhibitors (AChEIs), particularly for donepezil. In this review, the neuroprotective effects exhibited by currently commercialized AChEIs will be briefly discussed, together with the secondary mechanisms through which they could exert such effects. This review will focus also on particular classes of AChEIs, namely dual binding site AChEIs, which are being purposely designed to target Abeta aggregation and / or other biological targets that contribute to AD pathogenesis, thus constituting very promising disease-modifying anti-Alzheimer drug candidates.     

The potential for BACE1 inhibitors in the treatment of Alzheimer's disease

Alzheimer's disease (AD) is a debilitating neurodegenerative disease of the elderly characterized by the loss of memory and other cognitive functions. Currently marketed drugs for the treatment of this disease only offer symptomatic relief, and, consequently, there is a large unmet clinical need for disease-modifying therapies for the treatment of AD. Substantial research efforts are focused on inhibiting the proteases involved in the generation of the amyloidogenic beta-amyloid (A beta) peptide. One of these key proteases, beta-secretase, was identified as a novel transmembrane aspartic protease and named BACE1. Due to its pivotal role in A beta production, many pharmaceutical companies are actively pursuing the challenging task of developing BACE1 inhibitors for evaluation in the clinic.     

QSAR applications during last decade on inhibitors of acetylcholinesterase in Alzheimer's disease

This article reviews multi-criteria QSAR applications on Acetylcholinesterase inhibitors as palliative drugs for Alzheimer's Disease, published in the period 2001-2011. It includes QSAR models for different series of compounds, comparative studies, and advances in methodologies. This period is marked by a shift in focus from palliative treatment to pathogenesis. However, we believe that research into palliative treatment should continue. More comparative studies are desirable. In order to facilitate comparative and general studies on Acetylcholinesterase inhibitors, a standard experimental protocol for measuring an inhibitor's potency is needed. Finally, we recommend chemists to work closely with system and molecular biologists.     

Predicting relative binding free energies of tacrine-huperzine A hybrids as inhibitors of acetylcholinesterase

The binding of the 9-methyl derivative of tacrine-huperzine A hybrid to Torpedo californica acetylcholinesterase (AChE) has been studied by computational methods. Molecular dynamics simulations have been performed for the AChE-drug complex considering two different ionization states of the protein and two different orientations of the drug in the binding pocket, which were chosen from a previous screening procedure. Analysis of structural fluctuations and of the pattern of interactions between drug and enzyme clearly favor one binding mode for the tacrine-huperzine A hydrid, which mixes effectively some of the binding features of tacrine and huperzine A. The differences in inhibitory activity for a series of related derivatives have been successfully predicted by free energy calculations, which reinforces the confidence in the binding mode and its usefulness for molecular modeling studies. The same techniques have been used to make de novo predictions for a new 3-fluoro-9-ethyl derivative, which can be used to verify a posteriori the goodness of the binding mode. Finally, we have also investigated the effect of replacing Phe300 in the Torpedo californica enzyme by Tyr, which is present in the human AChE. The results indicate that the Phe330-->Tyr mutation is expected to have little effect on the binding affinities. Overall, the whole of results supports the validity of the putative binding model to explain the binding of tacrine-huperzine A hybrids to AChE.     

Cyclo-oxygenase-2 inhibitors: rationale and therapeutic potential for Alzheimer's disease

The newly introduced cyclo-oxygenase-2 (COX-2) inhibiting nonsteroidal anti-inflammatory drugs (NSAIDs) have been established as effective agents in treating arthritic conditions, while greatly reducing the gastrointestinal adverse effects of traditional NSAIDs. There are expectations that NSAIDs will be useful in the treatment of Alzheimer's disease (AD), and that COX-2 inhibitors might have a role. However, a recently reported clinical trial of a COX-2 inhibitor in AD indicated that it was neither protective nor did it accelerate the decline. The expectations were based on pathological evidence of inflammatory changes associated with AD lesions and epidemiological evidence of a reduced prevalence of AD in populations taking NSAIDs. They were supported by preliminary evidence showing efficacy of NSAIDs in treating patients with AD. These data are based on the use of traditional NSAIDs. Whether COX-2 inhibitors would be similarly effective was uncertain since COX-2 is constitutively expressed in neurons. Animal experiments suggest that COX-2 may be performing adaptive functions associated with normal neurons and protective functions associated with stressed neurons. These results emphasise that the appropriate target for NSAID trials in AD is COX-1, but they also indicate that there would be no contraindication to the use of those traditional NSAIDs which have mixed COX-1/COX-2 inhibiting activity.     

Huprine X is a novel high-affinity inhibitor of acetylcholinesterase that is of interest for treatment of Alzheimer's disease

Inhibitors of the enzyme acetylcholinesterase (AChE) slow and sometimes reverse the cognitive decline experienced by individuals with Alzheimer's disease. Huperzine A, a natural product used in traditional Chinese herbal medicine, and tacrine (Cognex) are among the potent AChE inhibitors used in this treatment, but the search for more selective inhibitors continues. We report herein the synthesis and characterization of (-)-12-amino-3-chloro-9-ethyl-6,7, 10,11-tetrahydro-7,11-methanocycloocta[b]quinoline hydrochloride (huprine X), a hybrid that combines the carbobicyclic substructure of huperzine A with the 4-aminoquinoline substructure of tacrine. Huprine X inhibited human AChE with an inhibition constant K(I) of 26 pM, indicating that it binds to this enzyme with one of the highest affinities yet reported. Under equivalent assay conditions, this affinity was 180 times that of huperzine A, 1200 times that of tacrine, and 40 times that of E2020 (donepezil, Aricept), the most selective AChE inhibitor currently approved for therapeutic use. The association and dissociation rate constants for huprine X with AChE were determined, and the location of its binding site on the enzyme was probed in competition studies with the peripheral site inhibitor propidium and the acylation site inhibitor edrophonium. Huprine X showed no detectable affinity for the edrophonium-AChE complex. In contrast, huprine X did form a ternary complex with propidium and AChE, although its affinity for the free enzyme was found to be 17 times its affinity for the propidium-AChE complex. These data indicated that huprine X binds to the enzyme acylation site in the active site gorge but interferes slightly with the binding of peripheral site ligands.     

Cholinesterase inhibitors for Alzheimer's disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disease and has become an urgent public health problem in most areas of the world. Substantial progress has been made in understanding the basic neurobiology of AD and, as a result, new drugs for its treatment have become available. Cholinesterase inhibitors (ChEIs), which increase the availability of acetylcholine in central synapses, have become the main approach to symptomatic treatment. ChEIs that have been approved or submitted to the US Food and Drug Administration (FDA) include tacrine, donepezil, metrifonate, rivastigmine and galantamine. In this review we discuss their pharmacology, clinical experience to date with their use and their potential benefits or disadvantages. ChEIs have a significant, although modest, effect on the cognitive status of patients with AD. In addition to their effect on cognition, ChEIs have a positive effect on mood and behaviour. Uncertainty remains about the duration of the benefit because few studies of these compounds beyond one year have been published. Although ChEIs are generally well tolerated, all patients should be followed closely for possible adverse effects. There is no substantial difference in the effectivenes of the various ChEIs, however, they may have different safety profiles. We believe the benefits of their use outweigh the risks and costs and, therefore, ChEls should be considered as primary therapy for patients with mild to moderate AD.     

Molecular modelling approaches to the design of acetylcholinesterase inhibitors: new challenges for the treatment of Alzheimer's disease

The interest for acetylcholinesterase as a target for the palliative treatment of Alzheimer's disease has been renewed in the last years owing to the evidences that support the role of this enzyme in accelerating the aggregation and deposition of the beta-amyloid peptide. A large amount of structural information on the acetylcholinesterase enzyme and of its complexes with inhibitors acting at the catalytic site, the peripheral binding site, or both is now available. Based on that, molecular modelling studies can be intensively used to decipher the molecular determinants that mediate the relationship between chemical structure and inhibitory potency. In turn, this knowledge can be exploited to design new compounds leading to more effective cholinergic strategies. At this point, inhibitors able to interact at the peripheral binding site are of particular relevance, as they might disrupt the interactions between the enzyme acetylcholinesterase and the beta-amyloid peptide. Therefore, these compounds might not only ameliorate the cholinergic deficit, but also be capable of slowing down the progression of the disease.